Aristide Economopoulos/The Star-LedgerPart of a flock of Canada Geese take flight over Denville during the 2000 Christmas Bird Count.

SOMERVILLE — Some birdwatchers liken it to a shopper unearthing an extraordinary bargain where it least was expected. Others say it's like stumbling across a trove of buried treasure. Tom Ostrand compared it to going to a baseball game and seeing a no-hitter.

Any way you want to describe it, the thrill of spotting a locally rare species is one of the fundamental reasons birdwatching is a hobby currently enjoyed by more than 50 million Americans, according to the U.S. Fish and Wildlife Service. And the National Audubon Society is inviting novice and experienced enthusiasts alike to participate in what it calls the largest citizen science project ever accomplished: the 111th annual Christmas Bird Count.

For the 63rd consecutive year, the initiative will take place at two local sites, both covering circular areas encompassing 15 square miles: the Great Swamp-Watchung Ridges count, slated for Saturday, Dec. 18, and the Raritan Estuary count, set for Dec. 26.

The local counts are just two of hundreds conducted annually across North and South America during a three-week period in December and January, as the results of each are recorded and uploaded into a massive online database that can shed light on long-term environmental and ecological shifts by illustrating how birds' habitats change over time.

"They should expect cold weather," Ostrand said of local participants, who have numbered between about 50 and 70 at each of the two sites during recent years. "And they should expect to see a lot of birds they've probably never seen before. That's what the exciting part is, I think."

Ostrand, a Metuchen resident and a software engineer for AT&T, is the coordinator of the Raritan Estuary count, which is centered in Edison and includes the Raritan River in Piscataway, Highland Park and New Brunswick; the Raritan Bayshore in Perth Amboy, South Amboy and Old Bridge; the Dismal Swamp, the Rutgers Ecological preserve and Cheesequake State Park.

Pete Axelrod is coordinating the Great Swamp-Watchung Ridges count, centered in New Providence and covering all of the Great Swamp, Lord Stirling Park, Morris County Outdoor Education Center, the Rahway River Greenway along Echo Lake and Lenape Parks, the Watchung Reservation, the South Mountain Reservation and several other parks and habitats in parts of Union, Morris, Somerset and Essex counties.

A combined total of more than 100 different species of birds have been spotted at the local counts during recent years, according to the National Audubon Society's digital archives, and along with hundreds and even thousands of Canada Geese, House Sparrows and Blue Jays, the annual outings also have produced plenty of rare finds. Between the two locations, 25 different species were recorded at just a single sighting apiece last year, the archives indicated.

"One single bird count doesn't mean a thing," explained Axelrod, a Long Hill resident who has coordinated the Great Swamp count for more than a decade and participated in it for more than a quarter-century. "But the trends tell you whether bids are coning or going, whether their habitats are expanding or shrinking. And it's all information that's 99 percent collected by your average, everyday citizen."

Each count site is divided into about a dozen sections, and each section has a team captain who is an experienced birder, the two coordinators explained.

Any number of participants can join any given count and fan out over as much territory as possible. Every bird seen is identified by species, and totals for all species are tallied. Some teams head out as early as 4 a.m. to listen for owls and other nocturnal species, Axelrod noted, and participants are free to decide for how long they'd like to be out and about — even if it's all day.

But more time out in the wilderness can make for some unforgettable moments, like spotting a Pileated Woodpecker: one of the world's largest woodpeckers, at least two have been spotted during each of the last five Great Swamp counts.

Managing New Jersey's Canada goose populationWildlife specialists from the United States Department of Agriculture were at Echo Lake Park in Mountainside to discuss and demonstrate techniques of managing the rising Canada goose population. Estimates have the resident population at around 92,000 birds in New Jersey. The Atlantic Flyway Council is suggesting the population be reduced by one half, around 45,000 birds statewide. (Video by Andre Malok / The Star-Ledger)

Similarly, exactly three Peregrine Falcons have been spotted during four of the last five Raritan Estuary counts, giving participants a look at one of the world's fastest living creatures, while multiple Bald Eagles also have been seen at both sites during each of the past four years.

Tallies of individual species often stay within certain ranges on a year-by-year basis, but both Axelrod and Ostrand said some wild variations can occur, frequently without explanation. More than 20,000 Common Grackles were spotted at the Great Swamp count in 2006 and in 2007, for example, then 26 were seen in 2008. Likewise, more than 5,000 Greater Scaups, a type of duck, were spotted at Raritan Estuary in 2007, but that number dropped by two-thirds in 2008, and just three were seen last year.

Ostrand said part of the Raritan Estuary counts covers privately owned land that only is opened to the public one day out of the year, giving participants a look into some quiet, isolated habitats rarely seen by human eyes.

"There are lots of old abandoned fields and ponds, and . . . the only way you get in there is with special permission (from the owners)," Ostrand said. "Doing the Raritan Estuary count I've found out about so many areas of Middlesex County that I always knew were there, but had never been through."

Axelrod singled out the American Kestrel, a colorful falcon, and the Pine Siskin, a type of vocal finch that enjoys hanging upside down to nibble on seeds in pine cones, as being two rare, exciting finds in the Great Swamp. Axelrod said he and others hope the local counts can bounce back from recent declines in participation so birds like those don't go unnoticed during future years.

"We're really looking to bring in new birders of any experience level," Axelrod said. "Just dress, dress, dress for the weather. In fact, overdress."